Zirconia (ZrO.sub.2), zircon (ZrSiO.sub.4), silica (SiO.sub.2) and chromium oxide (Cr.sub.2 O.sub.3) are well known in the art as neutral or acid refractory materials, and are used alone or in combination with other refractory materials. For example, a zirconia sintered body is stabilized or partially stabilized and can be utilized as a refractory material, and a chromia (chromium oxide) sintered body has also been utilized, particularly as a refractory material for the production of long glass fibers.
A refractory sintered body comprising zirconia and chromium oxide has not yet been produced, however, because it is difficult to obtain a high density sintered body thereof. Similarly, a refractory material comprising zircon and chromium oxide has not yet been produced because of difficulty of densification to sinter such a composition, and only zircon-based refractory materials containing from about 1% to 5% by weight of Cr.sub.2 O.sub.3 (which has been added to increase the corrosion resistance of the zircon-based refractory material) have been produced. In this case, however, since it is difficult to perform densification (that is, sintering to increase the density of the material), the performance cannot be improved to an extent that is sufficiently satisfactory.
Although a zircon refractory material has a small coefficient of thermal expansion, excellent heat impact resistance, and excellent abrasion resistance, it contains therein a number of pores, because it is difficult to perform densification of zircon alone (the densification cannot be achieved by high temperature heating because zircon is decomposed into zirconia and silica at about 1,500.degree. C. or more). Therefore, when the zircon refractory material is used as a refractory material, easily reactable substances such as a slag liquid enter the pores and react with zircon to form zirconia and low melting point compounds. Such low melting point compounds then melt and flow from the refractory material. Thus the structure of the refractory material is made brittle and is subject to corrosion.
A chromia refractory material comprising mainly Cr.sub.2 O.sub.3 has an excellent resistance to corrosion by liquid such a slag. However, it is inferior in heat stock resistance, because of its large coefficient of thermal expansion.
Although it would apparently be desirable to obtain ZrO.sub.2 -Cr.sub.2 O.sub.3 or ZrSiO.sub.4 -Cr.sub.2 O.sub.3 based refractory materials wherein the advantage of each component is exhibited compensating for the disadvantages of the other component, such refractory materials have not yet been produced in view of the foregoing difficulties.
A silica refractory material has a very small coefficients of thermal expansion at temperatures of about 600.degree. C. or more, and has excellent heat shock resistance and furthermore great high temperature strength. However, its corrosion resistance to various slag liquids except for acid slags are relatively poor because it is an acid refractory material. Recent studies on phase equilibrium have revealed that the foregoing drawback of the silica refractory material might be overcome by adding thereto Cr.sub.2 O.sub.3, and such addition of Cr.sub.2 O.sub.3 is effective particularly for the improvement in resistance to attack of those slags comprising mainly iron oxide. It also appears that the inferior heat shock resistance of the chromia refractory material can be improved by adding thereto silica, whose thermal expansion properties at high temperatures are low. Thus, it may be expected that formation of a dense structure comprising these two components, i.e., silica and chromia, would lead to the production of an excellent refractory material.
However, SiO.sub.2 and Cr.sub.2 O.sub.3 do not produce any new compound, and the eutectic point of the mixture is 1,720.degree. C., which is somewhat lower than the melting point of SiO.sub.2 (1,723.degree. C.), and sintering at high temperature causes evaporation of the Cr.sub.2 O.sub.3. Thus it has heretofore been difficult to densely sinter a compact consisting of the two components of SiO.sub.2 and Cr.sub.2 O.sub.3 alone, and thus such refractory materials have not yet been produced.
It is also known, as described in Japanese patent application (OPI) No. 96508/1979, that a compact consisting of Cr.sub.2 O.sub.3 alone can be densely sintered by heating it in carbon powder.